1. Field of the Invention
This invention relates to fluidized catalytic cracking during crude oil refining, and more particularly to improvement of conversion in fluidized catalytic cracking units supplied by feed streams contaminated with vanadium.
2. Description of the Prior Art
In general, a typical petroleum refinery includes a tank farm or storage area where crude petroleum and certain intermediates are held for optimum utilization by the refinery. The crude unit typically contains a desalter followed by a series of distillation columns whose principal function is to separate the crude oil feedstock into its various principal component fractions by differences in boiling point. The more volatile fractions are removed in the crude distillation tower leaving heavier gas oil fractions residual which are usually transported to a fluidized catalytic cracking (FCC) unit. Some of the bottom or residual fractions is also frequently included in the FCC feed. The FCC feed thus comprises various gas oil cuts and residual material and is routed to a pre-heat heat exchanger arrangement that comprises one or more heat exchangers and then a pre-heat furnace before introduction to the reactor of the FCC unit. Catalytic cracking typically employs a fluidized catalytic cracking unit which comprises a reactor containing a riser and a fluidized bed of catalyst, wherein the actual cracking process occurs, and a regenerator wherein catalyst is reactivated by burning coke and other residual materials from the surface of the catalyst. The products resulting from the cracking process, principally motor fuel blending stocks such as diesel fuel, jet fuel, kerosene and gasoline, are separated via fractionation columns.
In the reactor of the FCC unit, large hydrocarbon molecules are converted to smaller ones for use as transportation fuels. The degree to which the larger molecules are broken down to the smaller molecules is referred to as the FCC conversion or "yield". Improved conversion, particularly with respect to gasoline, is of extreme importance to the industry. Due to the high manufacturing costs and product values and the immense production scale involved, extraordinary efforts and expenses are undertaken for even slight improvements in conversion. The reaction is catalyzed by a fluidized catalyst; particularly a high zeolite content catalyst, typically about 25% to about 35% zeolite in a clay-like matrix.
Often, the FCC feed is contaminated with vanadium originating in the crude oil being refined. Typically, unsatisfactorily high vanadium levels are due mostly to the residuum component of the FCC feed, but gas oil from a coker unit, if present, can be a major culprit as well. The vanadium tends to coat the catalyst surface in the reactor and, in the regenerator, oxidize to a form in which it reacts readily with the catalyst to form low melting eutectics. As a result, the activity of the catalyst is impaired irreversibly by clogging of the catalyst pores. Accordingly, as vanadium levels build up in the reactor from the contaminated feed, FCC conversion is suppressed. See Vanadium Poisoning of Cracking Catalysts:Mechanisms of Poisoning and Design of Vanadium Tolerant Catalyst System, Wormsbecher et al., Journal of Catalysts 100, 130-137 (1986).
Conventionally, therefore, the problem of catalyst deterioration is addressed by a variety of techniques, none of which has been found to be wholly satisfactory. For example, the catalyst may be replaced or additional catalyst may be added, but this involves the obvious waste and costs. Additives also have been employed in certain attempts to fight vanadium contamination. Thus, tin compositions and solid particle vanadium scavengers have been used. However, tin compositions are very expensive and of only questionable efficacy. Since solid particle vanadium scavengers are solid, they are difficult to add to a feed stream, to disperse in the stream and to maintain them in dispersion through the stream. Moreover, special equipment is required for the handling and injection of the solid particles.
Many patents have been issued for improving hydrocarbon processes by alteration or modification of process conditions and/or equipment. Some patents have issued which relate to improved hydrocarbon processing by the use of process additive materials. Thus, British Application No. 985,180, published Mar. 3, 1965, discloses the use of alkaline earth metal salts of sulfonic acid or thiophosphonic acid to compatibilize feedstocks. Further, British Patent No. 1,602,098 discloses the addition of an alkaline earth metal oxide to a reactor to prevent deposit formation during the cracking of hydrocarbons. Also, German Patent No. 133,570 discloses the addition of alkaline earth metal sulfonates and sulfides to crude hydrocarbon refinery steams to inhibit coke deposition. Further, an overbase complex of calcium carbonate and a calcium salt of a sulfonic acid complexing agent and an overbase complex of magnesium oxide and the magnesium salt of a sulfonic acid have been used in treating crude oil in a refinery, although not for treatment of the FCC feed or for scavenging vanadium, but rather as pre-distillation column or fractionator additives.
Thus, in spite of the many changes and improvements to refinery systems over the years, the industry is still searching for techniques to improve conversion, for more satisfactory vanadium scavengers and for more satisfactory techniques for improving FCC catalyst life.